I Am The Cow, Destroyer Of Worlds (or: AT2018cow is one hell of a mysterious object)

[Don Alexander]

Please bear with me... Before I write anything serious, I just have to post this:

https://mysteriousuniverse.org/2018/...redible-speed/

The mind boggles, and the Woo woos.

Okay!! It seems there is no thread yet on AT2018cow. This is an Astronomical Transient (AT) discovered by the ATLAS survey on Mauna Kea (which has nothing to do with the Keck telescopes, and there is no "Keck observatory"...) - see http://www.astronomerstelegram.org/?read=11727. Opening this link should also list all other ATels posted.

The facts so far:

- The transient had a very rapid rise time. The probably tightest constraint is a non-detection (I estimate to 17th mag) by ASAS-SN 1.5 days before the discovery observation. It rose by more than three magnitudes over that time, which is quite unusual, at least for SNe that are this luminous.

- It is definitely associated with a face-on spiral galaxy with disturbed morphology at 60 Mpc (~ 200 Mly).

- The early emission which became bright so rapidly is NOT your typical SN emission but a very hot (about 25000 K) blackbody, excepting the narrow absorption lines from interstellar matter in the host and our Milky Way, the early spectra were featureless.

- Within some days, spectral features typical for a so-called broad-lined Type Ic SN (often associated with Gamma-Ray Burst [GRBs]) began to emerge, but the hot blackbody continues to dominate.

- The source has been detected brightly both in X-rays and in mm/sub-mm (and faintly in radio). This is quite atypical for SNe, and indicates a component which emits synchrotron emission, which usually implies matter accelerated to (ultra)relativistic velocities - as typically seen in GRBs. Except no one has detected an actual GRB from this event.

To summarize, this transient consists of three components. A (probably reasonably typical) broad-lined Type Ic SN, a relativistic synchrotron component (these two together form a "relativistic SN", a rare class of GRB-SN-like SNe without GRBs) - and then the very hot, extremely luminous, and seemingly hardly cooling blackbody emission, and THAT is the part which is unlike anything seen so far. There have been some GRB-SNe showing a similar (though less luminous) component at early times (SN 2006aj, for example) but this rose and decayed within about a day, and did not dominate the UV/optical emission as in this case.

Bright minds are probably already conjecturing models, but for now, the transient community is observing, observing, observing.

The SN, by the way, is not just luminous, but also bright in general, about 15th magnitude in the optical now, so amateurs with good scopes (20" or so) should be easily able to detect it.

[antoniseb]

Thanks for this. I'll keep an eye out for more details. Looks interesting.

[antoniseb]

I'm seeing more crackpot ideas on the internet claiming the AT2018COW as proof. Once again thanks Don Alexander for some facts before the nonsense arrived!

[slang]

Doesn't post often, but when he does... No mentions of GW detections, are there none or possibly not published yet?

[StupendousMan]

No mentions of GW detections, are there none or possibly not published yet?

LIGO is down for upgrades to its equipment (will start observing again in 2019), so it couldn't detect any GW associated with this event. VIRGO may have been active; their website doesn't provide any obvious status report, unfortunately. I certainly haven't heard any news from VIRGO on this event.

[eburacum45]

The event—or object—is referred to as “The Cow,” which seems a distinctly boring name for a violent, galaxy destroying explosion of unknown origin. It was detected as far brighter than an average supernova, and moving at an anomalously fast speed. The team of astronomers working the ATLAS telescope at the Keck Observatory reported the event in the Astronomer’s Telegram, where it sparked international curiosity and where the automated naming and cataloging system of the Telegram listed the report as “AT2018cow.” This is why we shouldn’t let computers name things.

I am constantly expecting deep space astronomy to turn up something like this; evidence of star-killer warfare, or a failed attempt to rip a wormhole into the next brane?
Or just another rare event, like a collision between two supermassive stars?

[Ken G]

The absence of gamma rays might not be so surprising-- they are typically beamed, perhaps moreso than the synchrotron emission and certainly moreso than the blackbody emission. Seems likely someone will suggest a fairly mundane model pretty soon! Though star killing certainly sounds more dramatic.

[ngc3314]

I turned the telescope over to some colleagues for a couple of hours to look at it about 10 nights back.

"I haven't double-checked the guider parameters tonight"
"No prob, we'll use short exposures, it's really bright"
"Let's offset to check HOLY COW, IS THAT IT?"

(I contend that would be a better origin for the name than the ATel designation permutations, anyway).

[Don Alexander]

Doesn't post often, but when he does... No mentions of GW detections, are there none or possibly not published yet?

Thanks for the honors.

As StupendousMan has stated, LIGO is down for upgrades. While I'm sure this source emitted gravitational waves upon explosion (as the is evidence it is some kin of core-collapse event), these were very likely undetectable at this distance (1.5 times the distance of GW 170817).

In recent days, more peculiarities have been posted:

- NuSTAR detected a hard X-ray spectral component which now seems to have disappeared, this could be a FOURTH emission component.

- The spectral features of the Ic SN have pretty much disappeared again! The spectrum as of now has gotten really weird, not explicable by a single blackbody + SN emission.

- Swift X-ray data have shown two bright flares which are not seen in the UV/optical data.

- The source seems to be decaying in some UV/opt bands but has come to a standstill in others. This goes even beyond the expected chromatic evolution of a cooling blackbody.

- multiple satellite missions now report... no detected GRB.

If anything, the Cow has become even more puzzling.

(I contend that would be a better origin for the name than the ATel designation permutations, anyway).

Actually, that's another error that is being propagated. The AT2018cow designation has been assigned by the TNS (Transient Name Server) at the time the discovery report was posted. It has nothing to do with ATel at all.

[ngc3314]

To show how bright it is - here's an RGB (optical) composite from data at the Jacobus Kapteyn Telescope on La Palma early on July 6.

[Don Alexander]

The Nordic Optical Telescope is now reporting the rise to prominence of a whole sequence of Helium emissions lines! It's rather unclear where these come from and whether they imply this is a Type Ib SN, not Type Ic...

So far, no Type Ib has ever been associated with relativistic emission.

[Jean Tate]

Hot off the arXiv press: "The Cow: discovery of a luminous, hot and rapidly evolving transient", Prentice+ (2018). Here's the abstract:

We present the ATLAS discovery and initial analysis of the first 18 days of the unusual transient event, ATLAS18qqn/AT2018cow. It is characeterized by a high peak luminosity (∼1.7×1044 erg s−1), rapidly evolving light curves (>5 mag rise in ∼3 days), hot blackbody spectra, peaking at ∼27000 K that are relatively featureless and unchanging over the first two weeks. The bolometric light curve cannot be powered by radioactive decay under realistic assumptions. The detection of high-energy emission may suggest a central engine as the powering source. Using a magnetar model, we estimated an ejected mass of 0.1−0.4 M⊙, which lies between that of low-energy core-collapse events and the kilonova, AT2017gfo. The spectra of AT2018cow showed a number of shallow features overlying a blackbody continuum. The spectra cooled rapidly from 27000 to 15000 K in just over 2 weeks but the positions of shallow bumps in the spectra did not evolve, suggesting that they are produced in a shell or are potentially emission features. Using spectral modelling, we tentatively identify some features as being due to He I and He II and rule out that the features in the spectra are due to most elements up to and including the Fe-group. The presence of r-process elements cannot be ruled out. If these lines are due to He, then we suggest a low-mass star with residual He as a potential progenitor. Alternatively, models of magnetars formed in neutron-star mergers give plausible matches to the data.

[profloater]

fascinating , I note that the luminosity converts to 1.7 10^37 Watts while our sun puts out a bit less than 4 10^26 watts

[Don Alexander]

And here is the next paper:

https://arxiv.org/abs/1807.06369

X-ray Swift observations of SN 2018cow

Supernova (SN) 2018cow is an optical transient detected in the galaxy CGCG 137-068. It has been classified as a SN due to various characteristics in its optical spectra. The transient is also a bright X-ray source. We present results of the analysis of ~62ks of X-ray observations taken with the Neil Gehrels Swift Observatory over 27 days. We found a variable behavior in the 0.3-10keV X-ray light curve of SN 2018cow, with variability timescales of days. The observed X-ray variability could be due to the interaction between the SN ejecta and a non-uniform circum-stellar medium, perhaps related to previous mass ejections from a luminous-blue-variable-like progenitor.

[2ndClassCitizen]

a violent, galaxy destroying explosion of unknown origin. It was detected as far brighter than an average supernova,

Isn't it logical that the most energetic cosmological events (Quasars, 1b supernovae, nebulae, and gamma ray bursts) arise from the most energetic reactions between particles? Matter and antimatter annihilations?

Big bang predicted that all antimatter was destroyed 13 billion years ago. Observations by NASA (Hubble) show that our galactic center is one half antimatter on one side and one half matter on the other. They also state that streams of antimatter positrons are created, and stream outward from the galactic center.

A precarious formation, a collision through the gc could upset this sensitive system, creating a cascade of annihilations.. Quasars are the galactic centers of galaxies.

Collisions by stars in arms of galaxies are rare (until they become old, full of dust, and arms begin to overlap).

When a matter star collides with streams of antimatter or another star composed of antimatter, one should expect a tremendous release of energy. This energy would quickly levels out when either matter or antimatter have been consumed leaving a remnant of the other.

[tusenfem]

Isn't it logical that the most energetic cosmological events (Quasars, 1b supernovae, nebulae, and gamma ray bursts) arise from the most energetic reactions between particles? Matter and antimatter annihilations?

Big bang predicted that all antimatter was destroyed 13 billion years ago. Observations by NASA (Hubble) show that our galactic center is one half antimatter on one side and one half matter on the other. They also state that streams of antimatter positrons are created, and stream outward from the galactic center.

A precarious formation, a collision through the gc could upset this sensitive system, creating a cascade of annihilations.. Quasars are the galactic centers of galaxies.

Collisions by stars in arms of galaxies are rare (until they become old, full of dust, and arms begin to overlap).

When a matter star collides with streams of antimatter or another star composed of antimatter, one should expect a tremendous release of energy. This energy would quickly levels out when either matter or antimatter have been consumed leaving a remnant of the other.

Dear 2ndClassCitizen
Welcome to CQ.
But be careful, you are treading here into "Against the Mainstream" waters. Please read the rules for this forum, which are linked in my signature.
Have fun here.

[2ndClassCitizen]

Everything i said i based on proven science, quotes of established theorists, and obbservations by NASA and Hubble. How is this against the "mainstream"?

The "against the mainstream" is code for "post your ridiculous ideas here" and there is a pre-assumption of incorrectness.

[slang]

Everything i said i based on proven science, quotes of established theorists, and obbservations by NASA and Hubble. How is this against the "mainstream"?

The "against the mainstream" is code for "post your ridiculous ideas here" and there is a pre-assumption of incorrectness.

You are violating rule 17 that prohibits taking threads off topic by arguing against moderator actions. Please don't do it again, and please take some time familiarizing yourself with this forum's rules. If you have a problem with a post, even a moderators, report it or use the feedback forum.

[Don Alexander]

Everything i said i based on proven science, quotes of established theorists, and obbservations by NASA and Hubble. How is this against the "mainstream"?

The "against the mainstream" is code for "post your ridiculous ideas here" and there is a pre-assumption of incorrectness.

To quote you:

"Observations by NASA (Hubble) show that our galactic center is one half antimatter on one side and one half matter on the other."

This is, yes, a "ridiculous idea."

There is clear evidence for sources of positrons in the Galaxy, but the actual amount of particles produced is vanishingly small compared to the matter mass present. There are no antimatter stars and there is especially no side of the galaxy that is made of antimatter.

Additionally, I see no relation to the actual topic of this thread.

[Swift]

To quote you:

"Observations by NASA (Hubble) show that our galactic center is one half antimatter on one side and one half matter on the other."

This is, yes, a "ridiculous idea."

There is clear evidence for sources of positrons in the Galaxy, but the actual amount of particles produced is vanishingly small compared to the matter mass present. There are no antimatter stars and there is especially no side of the galaxy that is made of antimatter.

Additionally, I see no relation to the actual topic of this thread.

Given the lack of a relationship to the topic of this thread, and the two moderator warnings, please don't debate 2ndClassCitizen's ideas.

[Don Alexander]

The Cow is still going strong in the radio and has now been pinpointed to micro-arcsecond accuracy by Very Long Baseline Interferometry:

http://www.astronomerstelegram.org/?read=11900

[Roger E. Moore]

Mooooo-re on the Mad Cow of Space.


https://arxiv.org/abs/1807.06369

X-ray Swift observations of SN 2018cow

L.E. Rivera Sandoval, T.J. Maccarone, A. Corsi, P.J. Brown, D. Pooley, J.C. Wheeler
(Submitted on 17 Jul 2018 (v1), last revised 3 Aug 2018 (this version, v2))

Supernova (SN) 2018cow is an optical transient detected in the galaxy CGCG 137-068. It has been classified as a SN due to various characteristics in its optical spectra. The transient is also a bright X-ray source. We present results of the analysis of ~62ks of X-ray observations taken with the Neil Gehrels Swift Observatory over 27 days. We found a variable behavior in the 0.3-10keV X-ray light curve of SN 2018cow, with variability timescales of days. The observed X-ray variability could be due to the interaction between the SN ejecta and a non-uniform circum-stellar medium, perhaps related to previous mass ejections from a luminous-blue-variable-like progenitor.

[Roger E. Moore]

https://arxiv.org/abs/1808.00969

The Fast, Luminous Ultraviolet Transient AT2018cow: Extreme Supernova, or Disruption of a Star by an Intermediate-Mass Black Hole?

Daniel A. Perley, Paolo A. Mazzali, Lin Yan, S. Bradley Cenko, Suvi Gezari, Kirsty Taggart, Nadia Blagorodnova, Christoffer Fremling, Brenna Mockler, Avinash Singh, Nozomu Tominaga, Masaomi Tanaka, Alan M. Watson, Tomás Ahumada, G. C. Anupama, Chris Ashall, Rosa L. Becerra, David Bersier, Varun Bhalerao, Joshua S. Bloom, Nathaniel R. Butler, Chris Copperwheat, Michael W. Coughlin, Kishalay De, Andrew J. Drake, Dmitry A. Duev, Sara Frederick, J. Jesús González, Ariel Goobar, Anna Y. Q. Ho, John Horst, Tiara Hung, Ryosuke Itoh, Mansi Kasliwal, Nobuyuki Kawai, Shrinivas R. Kulkarni, Brajesh Kumar, Harsh Kumar, Alexander S. Kutyrev, Tanazza Khanam, William H. Lee, Keiichi Maeda, Ashish Mahabal, Katsuhiro L. Murata, James D. Neill, Chow-Choong Ngeow, Bryan Penprase, Elena Pian, et al. (12 additional authors not shown)
(Submitted on 2 Aug 2018)

Wide-field optical surveys have begun to uncover large samples of fast (t_rise < 5d), luminous (M_peak < -18), blue transients. While commonly attributed to the breakout of a supernova shock into a dense wind, the great distances to the transients of this class found so far have hampered a detailed investigation of their properties until now. We present photometry and spectroscopy from a comprehensive worldwide campaign to observe AT2018cow (ATLAS18qqn), the first fast-luminous optical transient to be found in real time at low redshift. Our first spectra (<2 days after discovery) are entirely featureless. A very broad absorption feature suggestive of near-relativistic velocities develops between $3-8$ days, then disappears. Broad emission features of H and He develop after >10 days. The spectrum remains extremely hot throughout its evolution, and the photospheric radius contracts with time (receding below R<10^14 cm after 1 month). This behavior does not match that of any known supernova, although a relativistic jet within a fallback supernova could explain some of the observed features. Alternatively, the transient could originate from the disruption of a star by an intermediate-mass black hole, although this would require long-lasting emission of highly super-Eddington thermal radiation. In either case, AT 2018cow suggests that the population of fast luminous transients represents a new class of astrophysical event. Intensive follow-up of this event in its late phases, and of any future events found at comparable distance, will be essential to better constrain their origins.

[Roger E. Moore]

REVISED version...

https://arxiv.org/abs/1808.00969

The Fast, Luminous Ultraviolet Transient AT2018cow: Extreme Supernova, or Disruption of a Star by an Intermediate-Mass Black Hole?

Daniel A. Perley, Paolo A. Mazzali, Lin Yan, S. Bradley Cenko, Suvi Gezari, Kirsty Taggart, Nadia Blagorodnova, Christoffer Fremling, Brenna Mockler, Avinash Singh, Nozomu Tominaga, Masaomi Tanaka, Alan M. Watson, Tomás Ahumada, G. C. Anupama, Chris Ashall, Rosa L. Becerra, David Bersier, Varun Bhalerao, Joshua S. Bloom, Nathaniel R. Butler, Chris Copperwheat, Michael W. Coughlin, Kishalay De, Andrew J. Drake, Dmitry A. Duev, Sara Frederick, J. Jesús González, Ariel Goobar, Marianne Heida, Anna Y. Q. Ho, John Horst, Tiara Hung, Ryosuke Itoh, Jacob E. Jencson, Mansi M. Kasliwal, Nobuyuki Kawai, Shrinivas R. Kulkarni, Brajesh Kumar, Harsh Kumar, Alexander S. Kutyrev, Tanazza Khanam, William H. Lee, Keiichi Maeda, Ashish Mahabal, Katsuhiro L. Murata, James D. Neill, Chow-Choong Ngeow, et al. (15 additional authors not shown)
(Submitted on 2 Aug 2018 (v1), last revised 12 Aug 2018 (this version, v2))

Wide-field optical surveys have begun to uncover large samples of fast (t_rise < 5d), luminous (M_peak < -18), blue transients. While commonly attributed to the breakout of a supernova shock into a dense wind, the great distances to the transients of this class found so far have hampered a detailed investigation of their properties until now. We present photometry and spectroscopy from a comprehensive worldwide campaign to observe AT2018cow (ATLAS18qqn), the first fast-luminous optical transient to be found in real time at low redshift. Our first spectra (<2 days after discovery) are entirely featureless. A very broad absorption feature suggestive of near-relativistic velocities develops between 3-8 days, then disappears. Broad emission features of H and He develop after >10 days. The spectrum remains extremely hot throughout its evolution, and the photospheric radius contracts with time (receding below R<10^14 cm after 1 month). This behavior does not match that of any known supernova, although a relativistic jet within a fallback supernova could explain some of the observed features. Alternatively, the transient could originate from the disruption of a star by an intermediate-mass black hole, although this would require long-lasting emission of highly super-Eddington thermal radiation. In either case, AT 2018cow suggests that the population of fast luminous transients represents a new class of astrophysical event. Intensive follow-up of this event in its late phases, and of any future events found at comparable distance, will be essential to better constrain their origins.

[Roger E. Moore]

NEW NEWS!

https://arxiv.org/abs/1808.08492

Swift spectra of AT2018cow: A White Dwarf Tidal Disruption Event?

N. Paul M. Kuin, Kinwah Wu, Samantha Oates, Amy Lien, Sam Emery, Jamie Kennea, Massimiliano de Pasquale, Qin Han, Peter J. Brown, Aaron Tohuvavohu, Alice Breeveld, David N. Burrows, S. Bradley Cenko, Sergio Campana, Andrew Levan, Craig Markwardt, Julian P. Osborne, Mat J. Page, Kim L. Page, Boris Sbarufatti, Michael Siegel, Eleonora Troja
(Submitted on 26 Aug 2018)

The bright transient AT2018cow has been unlike any other known type of transient. Its large brightness, rapid rise and decay and initially nearly featureless spectrum are unprecedented and difficult to explain using models for similar burst sources. We present evidence for faint γ-ray emission continuing for at least 8 days, and featureless spectra in the ultraviolet bands -- both unusual for eruptive sources. The X-ray variability of the source has a burst-like character. The UV-optical spectrum does not show any CNO line but is well described by a blackbody. We demonstrate that a model invoking the tidal disruption of a 0.1 - 0.4 Msun Helium White Dwarf (WD) by a 10^5-10^6 Msun Black Hole (BH) could provide an explanation to most of the characteristics shown in the multi-wavelength observations. A blackbody-like emission is emitted from an opaque photosphere, formed by the debris of the WD disruption. Broad features showing up in the optical/infrared spectra in the early stage are probably velocity broadened lines produced in a transient high-velocity outward moving cocoon. The asymmetric optical/infrared lines that appeared at a later stage are emission from an atmospheric layer when it detached from thermal equilibrium with the photosphere, which undergoes more rapid cooling. The photosphere shrinks when its temperature drops, and the subsequence infall of the atmosphere produced asymmetric line profiles. Additionally, a non-thermal jet might be present, emitting X-rays in the 10-150 keV band.

[slang]

Why do these last two papers have such different estimates for the BH mass in the TDE matches? The first suggests 10^4 Msun, the other 10^6 Msun (which is SMBH range). Is such a large black hole in a spiral arm exceptional?

[Roger E. Moore]

THIRD revision....


https://arxiv.org/abs/1807.05965

The Cow: discovery of a luminous, hot and rapidly evolving transient

S. J. Prentice, K. Maguire, S. J. Smartt, M. R. Magee, P. Schady, S. Sim, T.-W. Chen, P. Clark, C. Colin, M. Fulton, O. McBrien, D. O`Neill, K. W. Smith, C. Ashall, K. C. Chambers, L. Denneau, H. A. Flewelling, A. Heinze, T. W.-S. Holoien, M. E. Huber, C. S. Kochanek, P. A. Mazzali, J. L. Prieto, A. Rest, B. J. Shappee, B. Stalder, K. Z. Stanek, M. D. Stritzinger, T. A. Thompson, J. L. Tonry
(Submitted on 16 Jul 2018 (v1), last revised 29 Aug 2018 (this version, v3))

We present the ATLAS discovery and initial analysis of the first 18 days of the unusual transient event, ATLAS18qqn/AT2018cow. It is characterized by a high peak luminosity (∼ 1.7 × 10 44 erg s −1), rapidly evolving light curves (> 5 mag rise to peak in ∼ 3.5 days), and hot blackbody spectra, peaking at ∼ 27000 K that are relatively featureless and unchanging over the first two weeks. The bolometric light curve cannot be powered by radioactive decay under realistic assumptions. The detection of high-energy emission may suggest a central engine as the powering source. Using a magnetar model, we estimated an ejected mass of 0.1−0.4 M sol, which lies between that of low-energy core-collapse events and the kilonova, AT2017gfo. The spectra cooled rapidly from 27000 to 15000 K in just over 2 weeks but remained smooth and featureless. Broad and shallow emission lines appear after about 20 days, and we tentatively identify them as He I although they would be redshifted from their rest wavelengths. We rule out that there are any features in the spectra due to intermediate mass elements up to and including the Fe-group. The presence of r-process elements cannot be ruled out. If these lines are due to He, then we suggest a low-mass star with residual He as a potential progenitor. Alternatively, models of magnetars formed in neutron-star mergers give plausible matches to the data.

[Roger E. Moore]

Two notes previously unnoted. Tidal disruption event? Also, no gamma radiation.


https://arxiv.org/abs/1809.05048

Photospheric Radius Evolution of Homologous Explosions

Liang-Duan Liu, Bing Zhang, Ling-Jun Wang, Zi-Gao Dai (Submitted on 13 Sep 2018)

Recent wide-field surveys discovered new types of peculiar optical transients that showed diverse behaviors of the evolution of photospheric properties. We develop a general theory of homologous explosions with constant opacity, paying special attention on the evolution of the photospheric radius R ph. We find that regardless of the density distribution profile, R ph always increases early on and decreases at late times. This result does not depend on the radiation and cooling processes inside the ejecta. The general rising/falling behavior of R ph can be used to quickly diagnose whether the source originates from a supernova-like explosion. The shape of the R ph evolution curve depends on the density profile, so the observations may be used to directly diagnose the density profile as well as the temperature profile of the ejecta. All the well-monitored supernovae show such a R ph rising/falling behavior, which is consistent with our theory. The recently discovered peculiar transient AT2018cow showed a continuous decay of R ph, which is disfavored to be of a supernova-like explosion origin. Our result therefore supports the interpretation of this transient as a tidal disruption event.


http://www.astronomerstelegram.org/?read=11956
"The H.E.S.S. array of imaging atmospheric Cherenkov telescopes was used to carry out follow-up observations of AT2018cow (ATel #11727) between the 3rd and 5th July 2018, for a total of 2.4 hours. H.E.S.S. observed the region around the source position (RA, Dec) = (244.00, 22.27) from ATel #11727. Preliminary off-site calibration and analysis with the data taken by the CT5 telescope (CT5 mono), searching for a point-like gamma-ray source from the source target, revealed no significant detection."

[Roger E. Moore]

...and/or it's a type Ib supernova, but a really weird one.

http://www.astronomerstelegram.org/?read=11836

[Roger E. Moore]

Found the webpage for the object with the IAU Supernova Working Group.

https://wis-tns.weizmann.ac.il/object/2018cow